Those internal combustion engines which have valves to control the entry of combustible gases and the escape of exhaust gases have a camshaft to precisely control the timing of the opening and the closing of both the intake and exhaust valves in a precise relationship to the position of the piston within the cylinder.
Conventional camshaft positioning and, hence, the opening and closing times of the valves are determined by adjustment of the cam sprocket to a position relative to a timing mark, which is determined relative to the piston location as dictated by the rotational position of the crankshaft of the engine. Once in the desired advanced/retarded position or dead center, the sprocket and camshaft each rigidly attached to the other thus are driven by the crankshaft and timing chain in a timed relationship. At optimum for one set of operating conditions of the engine, the timing of the camshaft is a compromise over the full spectrum of the engine speed (RPM's).
Internal combustion engines, such as automobile and truck engines, operate at widely varied engine speeds. Whenever operating at other than the optimum engine speed for which the camshaft is timed, the camshaft timing is less than optimum and in some cases will significantly degrade the engine performance from that obtainable by adjustments of the cam timing for that specific engine speed. If cam timing is adjusted to a different operating point (engine speed), then the newly selected operating point becomes the optimum operating point, and the operating performance of all other operating engine speeds are compromised to some extent.
The retarding of the cam allows the pressures in the cylinders to be reduced at the end of the exhaust stroke to a level approaching atmospheric pressure enhancing the entry of the fuel/air mixture, thereby improving performance.
In regard to any engines, the composition or mix and quantities of emissions varies with the operating speeds of the engine. Thus, an engine with the camshaft advanced to produce low engine speed torque improvement will produce excessive emissions at higher engine speeds because the valve timing should be retarded at higher speeds to optimize burning of the fuel/air mixture and to produce minimum exhaust emissions. For engines with the valve operations timed for optimum operations at higher engine speeds, the engine will not produce optimum performance and consequently will produce excess emissions whenever operating at lower operating speeds.
Previous attempts have been made to provide a solution to the camshaft timing dilemma described above, but none are known that have been completely successful.
U.S. Pat. No. 3,516,394 issued to R. G. Nichols disclosed a speed responsive device for controlling a two-part camshaft, using sliding centrifugal weights.
U.S. Pat. No. 4,177,773 issued to John R. Cribbs discloses a damped variable valve timing arrangement using springs to dampen vibration and extend life.
U.S. Pat. No. 4,615,313 issued to Yoshinori Tsumiyama discloses a centrifugally controlled decompression device to decompress the engine cylinders at low engine RPM.
U.S. Pat. No. 4,955,330 to Christian Fabi et al. discloses a cam timing device using sliding centrifugal weights to effect the camshaft advance and retardation. The weights are spring biased to a low speed, camshaft advanced position.
U.S. Pat. No. 5,056,478 to Thomas T. Ma discloses a hydraulic control for effecting camshaft timing.
U.S. Pat. No. 5,181,486 to John S. Gyurovits discloses a centrifugally actuated cam timing device which uses sliding weights to effect cam timing by interfacing the weights to phasing ramps.
U.S. Pat. No. 5,228,417 to Seinosuke Hara discloses a hydraulic valve timing adjustment control.
All of the above patents fail to provide a viable solution to the problem of controlling valve timing advance and retardation in response to engine speed, because the systems are either too complex and expensive or the devices do not provide positive drive connection between the element providing the drive force and sprocket to insure stable force transmission unaffected by vibrations overcoming the drive connection.
Internal combustion engines have been determined to provide a very wide range of loading on the timing chain driving the camshaft due to cyclical vibrations. Cyclical vibration is caused by those cyclical forces exerted on the lobe of the camshaft by the valve springs as well as the forces exerted on the camshaft through the timing chain and camshaft timing sprocket.
The forces of the valve springs are exerted at different locations on the cam profiles and in differing magnitudes due to the amount of cam rise at the point of lifter (follower) engagement with the cam. Further, while spread out substantially uniformly over time, the firing or ignition of the combustible mixture in the cylinders creates power or force peaks on the crankshaft and results in the cyclic vibration being transmitted through the timing chain and the timing chain sprocket on the camshaft and to the camshaft itself. The additive effect of the cyclical forces from the lifters and the cyclical forces from the timing chain is that the timing chain loading of the camshaft sprocket varies from positive loads to negative loads, particularly at a steady engine speed (constant engine RPM).
The constantly changing chain loading, particularly the negative loading, on the sprocket and/or the camshaft results in such destructive cyclical vibration that the timing chain may be damaged or destroyed by running the engine at constant RPM for an extended period of time. As is well known, whenever a timing chain breaks and the camshaft no longer is rotationally synchronized with the rotation of the crankshaft, the engine may experience significant damage or may be destroyed.
While complete elimination of the cyclic vibration which is so destructive to the timing of the camshaft may not be accomplished by any means because of the intermittent loading of camshaft and the crankshaft, the cyclic vibrational effects may be greatly mitigated by eliminating negative loading on the timing chain. If the negative timing chain loads are eliminated, then the chattering and jumping of the mechanical parts flowing from the intermittent negative loading also may be eliminated.